Replace all null with None in code base

[ericsun95]

As title.

[angelcervera]

Could you be more descriptive in the description?

• [X] In the Core module, nulls are used only in unit testing and in examples to open files. This is covered by #8
• [ ] In the Spark module, nulls are used to populate null values for Spark. Is here where you see the problem? I can check if it is working with None like null, but not sure.